1. Field of the Invention
The present invention relates to hydraulic power generation systems and, in particular, to an apparatus and method for generating power using a novel pseudo-osmosis process which efficiently exploits the osmotic energy potential between two bodies of water having different salinity concentrations.
2. Description of the Related Art
About 20% of the world's electricity is generated using hydropower. In the United States alone this resource accounts for about 12% of the nation's supply of electricity, producing more than 90,000 megawatts of electricity annually and meeting the needs of approximately 28.3 million consumers each year. Hydropower is a clean source of natural energy. Not only is it environmentally friendly (and even beneficial in terms of flood control, etc.), but it is also extremely cost-efficient. In the Northwest, for example, electricity from hydropower plants typically costs about $10 per megawatt hour to produce. This compares to about $60, $45 and $25 per megawatt hour to produce electricity at nuclear, coal and natural gas power plants, respectively.
However, current hydroelectric power plants are configured to recover only the energy component of water that is released as a result of elevational changes. In particular, hydroelectric power is typically generated by dropping 200-300 feet-head (61-91 m-head) of fresh water from a higher elevation to a lower elevation across a rotating turbine coupled to an electrical generator. The exhaust water flow is discharged at the lower elevation as energy-depleted fresh water run-off. But, as will be explained in more detail below, this fresh water run-off is not completely depleted of energy. In fact, the amount of remaining recoverable energy in the discharged fresh water can be as great as the equivalent of 950 feet-head (290 m-head) of water or more. To understand the nature and origin of this additional recoverable energy component it is helpful to look at how fresh water is created.
Fresh water begins as water vapor that is evaporated from the oceans by solar energy. This water vapor rises into the atmosphere whereupon it cools. Cooling causes the water vapors to condense into clouds, ultimately resulting in precipitation. Some of this precipitation occurs over land masses forming fresh-water lakes, accumulated snow-fall and an extensive network of associated rivers, streams, aquifers and other forms of water run-off. Ultimately, all or virtually all of this fresh water run-off makes its way back to the oceans, thus completing the cycle. In fact, throughout the world enormous quantities of fresh water is freely washed into the ocean each year as part of the naturally occurring water cycle and/or as part of various human interventions such as hydro-power facilities, municipal waste water treatment facilities, and the like.
The overall driving force behind the water cycle is solar energy radiating from the sun over millions of square miles of exposed ocean waters each day. It is this solar energy that causes evaporation of fresh water vapors from the relatively high-saline ocean waters. The amount of radiant solar energy absorbed in this process is enormous, representing approximately 2,300 kJ/kg (0.64 kW-hr/kg) of water evaporated. This absorbed energy causes a concomitant increase in the latent energy or enthalpy of the evaporated water. The vast majority of this latent energy (approximately 99%) is dissipated as heat energy into the atmosphere upon re-condensing of the water vapors into clouds. However, a small but significant portion of this latent energy (approximately 0.13%) remains stored within the resulting fresh-water precipitation. This remaining non-dissipated stored energy represents the so-called "free energy of mixing" (or "heat of mixing") of fresh water into sea water. Specifically, it is the additional incremental energy (beyond the energy of evaporation of pure water) that is required to separate the fresh water (or other solvent) from the salt water solution (or other solvent/solute solution).
The free energy of mixing reflects an increase in entropy of water (or other solvent) when it is transformed from its pure (fresh-water) state to its diluted (salt-water) state. It is a physical property of solvents, such as water, that they have a natural tendency to migrate from an area of relatively low solute concentration (lower entropy) to an area of relatively high solute concentration (higher entropy). Thus, an entropy gradient is created whenever two bodies of water or other solvent having differing solute concentrations are brought into contact with one another and begin to mix. This entropy gradient can be physically observed and measured in the well-known phenomena known as osmosis.
Osmosis is the flow of water through a selectively permeable membrane (i.e., permeable to water, but impermeable to dissolved solutes) from a lower concentration of solute to a higher one. It is a colligative phenomenon--that is, it is not dependent on the nature of the solute, only on the total molar concentration of all dissolved species. Pure water is defined as having an osmotic potential of zero. All water-based solutions have varying degrees of negative osmotic potential. Many references discuss osmotic potential in terms of pressure across a semi-permeable membrane since the easiest way to measure the effect is to apply pressure to the side of the membrane with higher negative osmotic potential until the net flow is canceled. "Reverse osmosis" is the phenomena that occurs when additional pressure is applied across a selectively permeable membrane to the point of reversing the natural flow-direction there-through, resulting in separation of the solvent from the solute.
But, just as it takes energy to separate an amount of fresh water from a body of salt water, such as through solar evaporation or using the well-known reverse-osmosis desalinization process, remixing the fresh water back into the ocean waters results in the release of an equal amount of stored energy (approximately 2.84 kJ/kg) of fresh water. If this source of latent stored energy could somehow be efficiently exploited, it could result in the production of enormous amounts of inexpensive electrical power from a heretofore untapped and continually renewable energy resource.
For example, if 30% of the average flow from the Columbia River could be diverted into a device that recovered this latent free energy of mixing or osmotic energy potential at 100% efficiency, it would generate 6,300 megawatts of power. To put this in perspective, the current hydroelectric facility of the Grand Coulee Dam on the Columbia River (the largest hydroelectric power plant in the United States and the third largest in the world) generates a peak output of 6,800 megawatts. If the flow from the Weber River into the Great Salt Lake could be diverted through such a device, it would generate 400 megawatts of power. Such a device would be of enormous benefit to people throughout the world, particularly those in remote regions where electrical power generation by conventional means may be difficult or impractical.
Various proposals have been made over the years for possible ways of commercially exploiting this attractive source of natural, renewable energy. For example, Jellinek (U.S. Pat. No. 3,978,344) proposed to pass fresh water through a semi-permeable membrane into a salt or brine solution. The resulting osmotic pressure differential across the membrane would then be used to eject a stream of salt water through an outlet orifice to drive a water wheel coupled to an electrical power generator to generate electrical power. Similarly, Loeb (U.S. Pat. No. 3,906,250) describes a method and apparatus for generating power utilizing pressure retarded osmosis through a semi-permeable membrane.
Each of the above approaches, like many others heretofore advocated, rely on a forward osmosis process utilizing a semi-permeable membrane to obtain useful work from the difference in osmotic potential exerted across the membrane. While such systems may have useful application on a small scale under certain limited conditions, full-scale commercial development and exploitation of such power-generation systems is hampered by the large membrane surface area required to achieve adequate flow rates and the expense and difficulty of maintaining such semi-permeable membranes. Although modem advances in synthetic materials have produced membranes that arc very efficient at rejecting brine solutes and are tough enough to withstand high pressures, such membranes are still susceptible to clogging, scaling and general degradation over time. For example, river water used as a fresh-water source would likely carry a variety of solutes and other suspended sediment or contaminants which could easily clog the membrane, requiring filtering and/or periodic cleaning. Treated effluent from a municipal waste-water treatment plant used as a fresh water source would present similar and possibly additional complications, making such approach commercially impractical.
Urry (U.S. Pat. No. 5,255,518) proposed an alternative method and apparatus for exploiting osmotic energy potential in a manner that does not utilize a semi-permeable membrane. In particular, Urry proposed the use of a specially formulated bio-elastomer. The bio-elastomer is selected such that it alternately and reversibly contracts or expands when exposed to different concentrations of a brine solution. A mechanical engine is proposed for converting the expansion and contraction motion of individual bio-elastomer elements into useful work. While such a system demonstrates the usefulness of the general approach, the proposed system utilizing bio-elastomer elements or the like is not readily suited for large-scale, low cost energy production. To produce useful energy on a commercial scale such a system would require a vast number of bio-elastic elements having very large surface area. Again, the exposed surface area would be subject to contamination and degradation over time, as with the membranes discussed above, making such a system prohibitively expensive to construct and maintain.
Assaf (U.S. Pat. No. 4,617,800) proposed another alternative apparatus for producing energy from concentrated brine in a manner that does not utilize a semi-permeable membrane or specially formulated bio-elastomer. In particular, Assaf proposed using a system of steam evaporation and re-condensation. In this approach steam is first generated by heating fresh water in an evaporator and passing the steam through a turbine to drive an electric generator. The condensed steam is then passed to a condenser wherein it is contacted with a flow of concentrated brine, generating heat from the heat of dilution of the brine. It is proposed that the evolved heat would then be transmitted though a heat-exchanger element back to the evaporator to generate steam from the fresh water. While this approach generally avoids the membrane and large surface area contamination problems discussed above, it is not ideally suited for large-scale, low cost energy production. This is because of the number and complexity of components involved and the need to heat and cool the fresh water in pressure sealed evaporator and condenser units. Such a system would be expensive to construct and operate on a commercial scale.
Thus, there remains a need for a method and apparatus for efficiently exploiting the osmotic energy potential between fresh water and sea water (and/or other solutions).